So-called “general-purpose plastics” such as polyethylene (hereinafter referred to as “PE”), polypropylene (hereinafter referred to as “PP”), polystyrene (hereinafter referred to as “PS”), and polyvinyl chloride (hereinafter referred to as “PVC”) are commonly used as materials for various daily-use products (such as bags, various wrappings, various containers, and sheets) and materials for industrial parts of automobiles and electrical products, daily necessities, miscellaneous goods, and the like, not only because they are available at very low prices of ¥100 or less per kilogram, but also because they are easy to mold and lighter in weight than metal and ceramics (one severalth the weight of metal or ceramics).
However, the general-purpose plastics suffer from such drawbacks as insufficient mechanical strength and low heat tolerance. As such, the general-purpose plastics do not sufficiently posses properties required of materials for use in various industrial products, e.g., mechanical products such as automobiles and electrical, electronic, and information products. Therefore, the general-purpose plastics are currently limited in scope of application. For example, PE typically has a softening temperature of approximately 90° C. Further, PP, which is considered to be comparatively high in heat tolerance, typically softens at 130° C. or less. Moreover, since PP is insufficient in transparency in comparison with polycarbonate (hereinafter referred to as “PC”) and polyethylene terephthalate (hereinafter referred to as “PET”) and PS, it suffers from such a drawback that it cannot be used as optical materials, bottles, or transparent containers.
On the other hand, so-called “engineering plastics” such as PET, PC, fluoroplastics (e.g., Teflon (registered trademark)), nylon, polymethylpentene, and an acrylic resin are excellent in mechanical strength, heat tolerance, transparency, and other properties, and typically do not soften at 150° C. Therefore, the engineering plastics are used as various industrial product materials and optical materials, which are required of high performance, for automobiles, mechanical products, and electrical products. However, the engineering plastics suffer from serious drawbacks. For example, they are sold at very high prices of several hundred yen per kilogram to several thousand yen per kilogram. Further, the engineering plastics are very environmentally unfriendly because it is difficult or impossible to convert them back into monomers for recycling.
Therefore, if the material properties such as mechanical strength, heat tolerance, and transparency of the general-purpose plastics are so dramatically improved that the general-purpose plastics can replace the engineering plastics and even metal materials, it becomes possible to greatly reduce costs of various industrial products and daily-use products made of polymers and metals, greatly save energy through a reduction in weight, and improve user-friendliness. For example, if PP becomes able to replace PET which is currently used as bottles for beverages such as soft drinks, it becomes possible to greatly reduce costs of bottles. Moreover, since it is possible but is not easy to recycle PET into monomers, used PET bottles are cut, reused once or twice for low-quality applications such as clothing fibers and films, and then discarded. Meanwhile, easiness of recycling PP into monomers allows complete recycle of PP, thus bringing about merit that makes it possible to reduce the consumption of fossil fuels such as oil and the generation of carbon dioxide (CO2).
In order to improve the properties such as mechanical strength, heat tolerance, and transparency of the general-purpose plastics so that the general-purpose plastics can replace the engineering plastics and metals, it is necessary to remarkably increase the proportion of crystals in PP or PE (crystallinity), or more preferably, to prepare a crystalline substance that is purely crystalline and contains not much amorphous PP or PE. In particular, PP has the advantage of being higher in mechanical strength and heat tolerance than PE. Therefore, PP is a very promising, important polymer that has been maintained at high annual rates of increase in production of several percent.
One method known to improve the crystallinity of a polymer is to cool a melt of the polymer at a slow rate. This method, however, cannot sufficiently increase the crystallinity at all. Further, this method causes significant deterioration in productivity of products, and this method also increases crystal grain size to a bulky size, thus causing a decrease in mechanical strength. Another method proposed to increase the crystallinity is to cool the melt of the polymer under high pressure. This method, however, requires pressing of the melt of the polymer at several hundred atm or greater. Thus, this method is only experimentally possible, but not feasible in industrial production due to complicate production apparatus design and high production cost. Thus, this method is difficult to adopt practically. Another method known to improve the crystallinity of the polymer is to add a nucleating agent to the polymer melt. However, this method currently suffers from such drawbacks (a) inevitable contamination of the nucleating agent as impurities, (b) an insufficient increase in crystallinity, and an increase in cost due to the nucleating agent being much higher in cost than the resin. In conclusion, there is currently no method completed to dramatically improve the crystallinity of a polymer such as a general-purpose plastic and to produce a crystalline substance of a polymer.
Incidentally, many studies have shown that the polymer melt (isotropic melt) in which molecular chains take random conformation (so called “random coil”) is crystallized under shear flow to sparsely generate a combination of shish crystal form and kebab crystal form in the polymer melt (see Non-patent Literature 1). The shish crystal form is a fiber-like crystal of several μm in thickness and is oriented along the flow. The kebab crystal form is a lamination of thin-film crystal and amorphous skewered through the shish crystal form. This form is referred to as “shish-kebab, meaning “skewer” and “meat” of skewered grilled-chicken (Japanese “Yakitori”)”.
In the production of shish-kebab form, only the shish form is created locally in an initial period. The shish form has an Extended Chain Crystal (ECC) structure in which straightly-elongated molecular chains are crystallized (see Non-patent Literature 5). On the other hand, the crystal portion of the kebab form has a Folded Chain Crystal (FCC) structure in which the molecular chains are folded at a surface of the thin-film crystal. How the shish-kebab form is produced has not been explained in terms of molecular theory, because it has not been studied kinetically. The FCC is a thin-film crystal (called a lamellar crystal) which is most widely seen among polymer crystals. Moreover, it is widely known that injection molding forms a “skin” (which is a thin crystalline film of several hundred μm thickness) on surface, and a “core” inside. The core is an aggregate of “laminated structures (laminated lamellar structures)” in which a folded chain crystal and amorphous are laminated. (see Non-patent Literature 6). The skin is formed from shish-kebab form, but the shish is formed only sparsely. Production mechanism of the skin structure has been totally unknown in the lack of kinetic study thereon.
The inventors of the present invention are pioneers to study the production mechanism of the shish form kinetically, and found the mechanism of the local formation of the shish form in the melt: at a boundary with heterogeneity, some molecular chains in the melt attain liquid crystal orientation because the molecular chains are elongated due to “topological interaction” with the boundary, and the melt become “Oriented melt” (e.g., see Non-patent Literatures 2 and 3). Here, the “topological interaction” is an effect that “string-like polymer chains characterized by a one-dimensional topology (mathematical topology) pull each other and slide on each other under flow field because they are entangled”. The topological interaction is well-known as a characteristic interaction among polymers. The inventors of the present invention are first to report a theory of the topological crystallization mechanism of polymers, explaining how the ECC and FCC are formed. This theory is called “sliding diffusion theory” and recognized worldwide (see Non-patent Literature 7).
Moreover, the inventors of the present invention discovered the mechanism of generation of a “spiralite” during the crystallization of a polymer at a low shear strain rate of 0.01 to 0.1 s−1 under a shear flow field, clarified the mechanism of generation, and thus became the first to experimentally verify that molecular chains are elongated at a boundary with heterogeneity to form the oriented melt, thereby advocating a universal mechanism in which nucleation and growth speed are remarkably accelerated (see Non-patent Literature 4).
Consequently, it can be said that the crystallization of a polymer will be facilitated and the crystallinity thereof can be enhanced if the polymer melt can be turned wholly into the oriented melt. The “polymer melt turned wholly into the oriented melt” here is referred to as “bulk oriented melt”. Furthermore, it is expected that if the polymer melt can be wholly crystallized while being kept as the oriented melt, a crystalline substance in which a majority of molecular chains of the polymer are oriented (such a crystalline substance being referred to as bulk “oriented polymer crystalline materials”) can be produced. In this case, the nucleation is facilitated remarkably and changed drastically into “homogeneous nucleation” by which an indefinitely large number of nuclei are formed between molecular chains without addition of a nucleating agent. This eliminates impurity contamination and allows the crystal size to be on the order of nanometers. It is also expected that this leads to polymers with high transparency and dramatic improvement in mechanical strength and heat tolerance. The term “homogeneous nucleation” here means a case where according to a well-known classical theory of nucleation, nucleation spontaneously occurs without the aid from a foreign material such as a nucleating agent (see Non-patent Literature 8). On the other hand, a case where nucleation occurs on a surface a foreign body such as a nucleating agent with the aid of the foreign body is called “heterogeneous nucleation”. Conventionally, crystallization of every substance from the bulk melt has been “heterogeneous nucleation”.